In CID 1432593, Coverity complains that the result of qdict_crumple()
might leak if it is not a dictionary. This is not a practical concern
since the test would fail immediately with a NULL pointer dereference
in qdict_size().
However, it is not nice to depend on qdict_size() crashing, so add an
explicit assertion that that the crumpled object was indeed a dictionary.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>